The present invention relates to an apparatus having a rotor, such as a helicopter, and more particularly, it relates to a system and method for reducing vibration or stress induced in the apparatus by the rotor.
Rotors which propel helicopters and other propeller-driven aircraft induce low-frequency vibration in the structure supporting the rotor. The vibrations occur at frequencies starting at the shaft rotation rate and occur at many harmonics of the rotation rate. These vibrations result in structural damage, crew fatigue, and ultimately become one of the primary factors limiting the maximum forward speed of the aircraft. Similar vibrations are produced by fans and compressors in fixed installations as well as by marine propellers.
A primary source of the vibration problems is non-uniform airloads on the blades, although mass imbalance is not uncommon. Aerodynamic anomalies, however, tend to develop recurrently due to blade wear, damage, deformation, etc. The aerodynamic and certain mass and stiffness distribution anomalies have often been called "tracking faults", since a primary observable feature of the uneven airloads or mass distribution is a tendency for the blades to flap and/or deflect unevenly, and thus follow different "tracks." The troublesome manifestation of the aerodynamic and mass imbalance, however, is usually the 1/REV and N/REV vibration, and not the track deviations themselves.
All helicopter rotor trim balancing methods currently employed rely, at least in part, upon making the track of each blade identical. Such optical methods, however, utilize bulky equipment which relies upon an operator in the co-pilot seat and procedures which require considerable flight time. Furthermore, optical methods cannot always "see" the blades during a complete revolution and thus cannot be expected to achieve perfect aerodynamic trim.
Mechanical balancing of rotors with mass imbalance can in many cases be performed with a single accelerometer and a shaft-phase reference sensor. However, uneven airloads cannot be fully diagnosed and corrected with such a technique. Other techniques used to perform the rotor smoothing function rely upon optical tracking in conjunction with one to four accelerometers. Every known rotor smoothing system, however, processes vibration data in such a way that there is an inherent ambiguity in the interpretation of the signatures. This ambiguity comes about because the number of channels processed simultaneously is inadequate to fully separate translational and rotational acceleration components at a given point. Thus, the motion of the helicopter (and in particular the rotor support) in response to a rotor anomaly is incompletely specified in known systems. Furthermore, no known system can deduce the corrections needed from the Fourier coefficients of the motion related to each anomaly.
It is therefore a principal object of the present invention to provide a system and method for minimizing vibration over the full operating range of a rotor of an apparatus which utilizes, as sensed input, only vibration measurements in the structure supporting the rotor and a measure of the shaft position.
It is a further object of the present invention to provide a system and method for minimizing vibration over the full operating range of a rotor of an apparatus that will compensate for both aerodynamically and mechanically induced vibration.
Still another object of the present invention is to provide a system and method for minimizing vibration over the full operating range of a rotor which will provide continuous monitoring and computation of corrections.
Yet another object of the present invention is to provide a system and method for minimizing vibration over the full operating range of the rotor which computes the aerodynamic and mechanical imbalance on any type of rotor through full characterization of the response of the supporting structure and a detailed dynamical model of the mechanics of the rotor.
It is a still further object of the present invention to provide a system and method for minimizing vibration over the full operating range of a rotor which prescribes changes needed in rotor settings through calculations and/or measurements of the effect of mechanical adjustments on the motion of the structure supporting the rotor.
Another object of the present invention is to provide a system and method for minimizing vibration over the full operating range of a rotor that allows a user to utilize the correction algorithms through controlled tests in which blade settings are adjusted and the effect on the structure recorded and interpreted.